Preparation of cyclobutarenes via the steam pyrolysis of aromatic derivatives

ABSTRACT

Cyclobutarenes are prepared by pyrolyzing a suitable benzene, naphthalene, or pyridine derivative in the presence of an amount of steam effective to substantially reduce the partial pressure of the pyrolyzing compound.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for preparingarylcyclobutenes, more commonly referred to as cyclobutarenes.

Cyclobutarenes, and in particular benzocyclobutene, are importantintermediates for the preparation of monomeric and polymericcompositions. U.S. Pat. No. 4,540,763 discloses that biscyclobutarenescan be processed to prepare polymeric compositions. These compositionsexhibit thermal stability at temperatures exceeding 250° C., chemicalresistance to most conventional solvents, good mechanical and electricalproperties, and low sensitivity to water. They are useful in advancedcomposites, adhesives, structural laminates, matrix resins, andplanarization resins for the electronics and aerospace industries.

As disclosed in Schiess et al., Tetrahedron Letters, pp 4569-4572(1978), cyclobutarenes have been prepared by the flash vacuum pyrolysisof an ortho-methyl-benzylchloride derivative. For example, the flashvacuum pyrolysis of α-chloro-ortho-xylene (ACOX) will yieldbenzocyclobutene. The pyrolysis is performed under vacuum to achieve alow partial pressure of the reactant because the conversion of thereactant to the cyclobutarene prepared increases as the partial pressureof the reactant decreases.

The flash vacuum pyrolysis process has three main problems associatedwith it. First, expensive refrigeration equipment is required tocondense the product and other expenses are required to operate undervacuum. Second, the process forms coke or tar on reactor internals andtherefore prevents ecnonmical continuous operation. Third, hydrochloricacid, which is produced as a byproduct of the pyrolysis in someinstances, is highly corrisive to the vacuum and refrigerationequipment.

Another method of decreasing the partial pressure of the reactant isdisclosed in U.S. Pat. No. 4,570,011. This method uses a mixture of thereactant and an inert solvent, such as xylene, to decrease theconcentration of the reactant during pyrolysis and therefore decreaseits partial pressure. However, this method requires the use of a largequantity of solvent which must be separated from the cyclobutarene andrecovered. More significantly, the operating pressure must still bereduced to a preferred pressure between 25 mm and 35 mm of mercury inorder to achieve a desirable yield of the cyclobutarene.

In view of the deficiencies of the prior art, a process for preparingcyclobutarenes with acceptable yields at substantially atmosphericpressure is needed. Additionally, a process that sufficiently reducescoke or tar formation on reactor internals to allow continuous operationis needed. Furthermore, a process that aids in separating hydrochloricacid or any other acid produced during the reaction and does not requirea large quantity of solvent would be highly desirable.

SUMMARY OF THE INVENTION

The present invention improves the known process of preparing acyclobutarene by pyrolyzing a benzene or a naphthalene substituted withany of halomethyl, hydroxymethyl, acetoxymethyl ortrifluoroacetoxymethyl and either methyl or substituted methyl orthothereto having at least one hydrogen on the alpha carbon. Theimprovement comprises conducting the pyrolysis in the presence of anamount of steam effective to substantially reduce the partial pressureof the pyrolyzing compound.

Surprisingly, the steam does not hydrolyze the reactant during pyrolysisto reduce the yield of desired cyclobutarene, despite high pyrolysistemperatures. The steam functions as a diluent to reduce the reactantpartial pressure so that vacuum operation is unnecessary. It alsoreduces coke or tar formation on reactor internals relative to the cokeor tar formation exhibited during vacuum operation and allows economicalcontinuous operation. When the steam condenses following the reaction,the aqueous phase formed contains the byproduct acid which can be easilyseparated from the cylobutarene. The improvement provides a practicalprocess for preparing cyclobutarenes at acceptable yields.

The cyclobutarenes prepared by the process of the present invention arenecessary intermediates for the patented polymer compositions of U.S.Pat. No. 4,540,763 and for other thermally stable polymer compositions.

DETAILED DESCRIPTION OF THE INVENTION

The pyrolyzing compounds of this invention are known in the art. Theyare benzene or naphthalene compounds substituted with any of thefollowing: ##STR1## either methyl or substituted methyl in a positionortho to the halomethyl, hydroxymethyl, acetoxymethyl, ortrifluoroacetoxymethyl substituent. The term "substituted methyl" refersto substituent formed by replacing at least one hydrogen on methyl withany atom or radical, including but not limited to atoms or radicals suchas halo, lower alkyl, nitro, and cyano. The substituted methylsubstituent must have at least one hydrogen on the alpha carbon.

The preferred halomethyl substituent is chloromethyl. The preferredsubstituted methyl substituents are lower alkyl, such as ethyl andpropyl; and halomethyl. The most preferred substituted methylsubstituent is chloromethyl.

The type of pyrolysis reaction necessary to prepare the cyclobutarenesof this invention depends on the particular pyrolyzing compound. If thepyrolyzing compound is substituted with halomethyl, then it undergoesdehydrohalogenation to form cyclobutarenes. Dehydrohalogenation is areaction in which a hydrogen halide, such as hydrogen chloride orhydrogen bromide, is removed from the pyrolyzing compound.Dehydrohalogenation is illustrated as follows: ##STR2## If thepyrolyzing compound is substituted with hydroxymethyl, then it undergoesdehydration to form cyclobutarenes. Similarly, if the pyrolyzingcompound is substituted with acetoxymethyl or trifluoroacetoxymethyl,then it undergoes dehydrocarboxylation. Dehydrocarboxylation is areaction in which a carboxylic acid, such as acetic acid ortrifluoroacetic acid, is removed from the pyrolyzing compound.Dehydrocarboxylation is illustrated as follows: ##STR3##

For purposes of describing this invention, a cyclobutarene is a benzeneor naphthalene compound to which is fused one or more cyclobutane ringsor one or more substituted cyclobutane rings.

U.S. Pat. No. 4,570,011; Scheiss et al., Tetrahedron Letters, 46, pp.4569-72, (1978); and Scheiss et al., Tetrahedron Letters, Vol. 23, No.36, pp. 3365-68, (1982); disclose substituted benzene and naphthalenecompounds of this invention that can undergo dehydrohalogenation,dehydration, or dehydrocarboxylation to form cyclobutarenes.

The references disclose that the aromatic ring can be furthersubstituted with at least one substituent stable to the pyrolysisconditions, including but not limited to substituents such as methyl,methoxy, methoxycarbonyl, nitro, chloro, bromo, and iodo. The mostpreferred pyrolyzing compound is ACOX, which when pyrolyzed will yieldbenzocyclobutene.

The pyrolyzing compounds of this invention can be prepared in situ fromredily available raw materials. For example, ortho-xylene (o-xylene) canreact with chlorine in situe to form ACOX, which can further react whenpyrolyzed to form benzocyclobutene.

The reaction conditions that define this improved process are the moleratio of steam to reactant (the reactant is the pyrolyzing compound ofthis invention), the reactor temperature and pressure, and the liquidhourly space velocity of the reactant through the pyrolysis reactor. Thereaction conditions should be adjusted to achieve the highest possibleconversion of reactant to desired product without excessive reactorvolume and to reduce the formation of tar or coke in the reactor duringthe pyrolysis.

The mole ratio of steam to reactant in preferred embodiments ranges fromabotu 5:1 to about 100:1, with a more preferred range from about 10:1 toabout 40:1. If the ratio falls below about 5:1, then the conversionbecomes unacceptable because of the increase in reactant partialpressure. If the ratio exceeds about 100:1, then the reactor volumebecomes excessive and the cost of producing steam and disposing ofcondensate become burdensome.

The reactor temperature is similar to the temperature required for flashvacuum pyrolysis. It can range from about 400° C. to about 800° C., witha preferred range from about 550° C. to about 700° C. Temperatures below400° C. require excessive reactor volume while temperatures above 800°C. enhance the likelihood of coke or tar formation. The reactor pressurecan range from about 0.1 atmosphere to substantially atmospheric.Subatmospheric pressures are advantageous since conversion increaseswith a further reduction in reactant partial pressure. However, reactorpressures below about 0.1 atmosphere would require refrigerationequipment to condense the reactor effluent and are thus less attractive.substantially atmospheric pressure is most convenient. Higher pressurescan be employed, but would cause an undesirable increase in reactantpartial pressure.

The liquid hourly space velocity is selected empirically based on theprocess conditions described above to maximize the conversion ofreactant to desired product. In preferred embodiments it ranges fromabout 0.5 volume of liquid per volume of reactor per hour (v/v/hr) toabout 10 v/v/hr. The more preferred range is from about 0.8 v/v/hr toabout 3.0 v/v/hr.

The pyrolysis reaction can occur in a reactor of any shape or form thatcan tolerate temperatures exceeding at least 400° C. for the requiredreaction time. The preferred reactor configuration has a minimum ofreactor internals upon which tar or coke can form and allows theresidence time distribution to approach plug flow. One configurationthat embodies these characteristics is a tubular reactor with a lengthto diameter ratio as great as practically possible. The preferredtubular reactor has a length to diameter ratio greater than 10:1.

The reactant and the steam can be fed to the reactor in any manner. Theycan be fed through different entry ports, or if desired, they can bepremixed before entering the reactor. Preferably, the reactant isvaporized and combined with the steam before entering the reactor. Thecombined flowrate should remain as constant as possible to approach plugflow. Alternatively, water instead of steam can be fed to the reactorand subsequently vaporized in the reactor.

In a preferred embodiment of this invention, a uniform temperature ismaintained within the reactor during pyrolysis. The elimination oflocalized "hot spots" reduces the formation of coke and tar on reactorinternals and prevents the occurrence of secondary reactions. One methodof maintaining a uniform temperature is to position the reactor in afluidized bed of fine powder, such as alumina, silica, or magnesia, andthen apply the necessary heat to the fluidized bed. The fluidized beddistributes the heat and prevents significant fluctuations in reactortemperature.

In another embodiment of this invention, the reactant is vaporized andpreheated with the steam to near reaction temperature in a preheaterbefore entering the reactor. The preheater configuration should effectsufficient mixing between the vaporized reactant and the steam toprovide a uniform composition before entering the reactor.

Following the reaction, the products are condensed and form an organicphase and a wastewater phase. The products can be condensed in aconventional shell-and-tube heat exchanger. The organic phase generallycontains the desired cyclobutarene and the wastewater phase containswater and possibly either the byproduct hydrogen halide or carboxylicacid, both of which would have been substantially diluted in water. Thetwo phases can easily be separated by decantation.

In a preferred embodiment, condensed reactor effluent contacts thevaporized reactor effluent to quickly condense and cool the vaporizedreactor effuent before it enters the heat exchanger. Rapid condensationand cooling of the reactor effuent reduces the formation of secondaryproducts.

The improved process of this invention enables the skilled artisan toprepare cyclobutarenes with acceptable yields at atmospheric pressure.An acceptable yield of cyclobutarene is greater than about 20 weightpercent. "Yield" is defined as the percent of reactant fed to thereactor that is converted to the desired cyclobutarene. Thecyclobutarenes of this improved process are necessary intermediates forpatented polymeric compositions prepared from biscyclobutarenes andother thermally stable polymer compositions.

The following examples are illustrative only and do not limit the scopeof this invention.

EXAMPLE 1

A pyrolysis reactor is fabricated from a quartz tube having an insidediameter of 12 millimeters (mm) and a length of 53 centimeters (cm). Thereactor is placed in an electric furnace and is heated to an averagetemperature of 606° C. 1.915 Grams per minute (g/min) of liquid ACOX atambient temperature, 9.354 g/min of superheated steam at substantiallyatmospheric pressure, adn 62 standard cubic cemtimeters per minute(SCCM) of nitrogen are fed cocurrently through different entry portsinto the top of the reactor. The reaction mixture flows downward throughthe tubular reactor and the vaporized effluent exits at the bottom ofthe reactor. The reactor is maintained at substantially atmosphericpressure. The reactor effluent is condensed and cooled with water in ashell-and-tube heat exchanger and is allowed to decant in a productreceiver. When a sufficent quantity of collected effluent is available,it is pumped out of the product receiver and is contacted with thevaporized reactor effluent to quickly condense and cool the effluentbefore it enters the heat exchanger.

After 144 minutes at these process conditions, the feeds are stopped.The collected effluent forms an organic phase and a wastewater phase inthe product receiver. 139 Grams of the organic phase is separated fromthe wastewater phase in a separatory funnel. The organic phase isanalyzed by gas chromatography using para-bromotoluene as an internalstandard. The analysis shows that 46.5 percent of the ACOX reacted andthat 45.0 percent of the ACOX that reacted formed benzocyclobutene.Therefore, the yield of ACOX to benzocyclobutene is 20.9 percent.

EXAMPLE 2

A tubular reactor is fabricated from coiled quartz tubing having aninside diameter of 15 mm and a length of 600 cm. The reactor ispositioned in a fluidized bed of alumina powder. An electric furnace isused to heat the fluidized bed and to maintain the reactor temperatureat 641° C.

37.1 Grams/min of liquid ACOX at ambient temperature, 91.3 g/min of lowpressure steam superheated to 180° C., and 50 cm3/min of nitrogen at 20°C. and 0.98 atm are initially fed concurrently through different entryports into the top of a quartz preheater. The quartz preheater is packedwith 0.25 inch ceramic Intalox saddles and is heated to 550° C. in anelectric furnace.

After the feeds are heated and vaporized in the preheater, they are fedinto the top of the reactor. The reactor is maintained at substantiallyatmospheric pressure and the calculated average partial pressure of ACOXfed to the reactor is 37.6 mm mercury. The feeds pass through the coiledreactor. The reactor effluent is condensed and cooled in ashell-and-tube heat exchanger. The condensed effluent is collected in aproduct receiver.

After 32.3 hours, the feeds to the preheater are stopped. The collectedeffluent forms an organic layer and a wastewater layer in the productreceiver. The organic layer is separated by decantation and analyzed bygas chromatography. The analysis shows that 45.4 percent of the ACOXreacted and that 65.6 percent of the ACOX that reacted formedbenzocyclobutene. Therefore, the yield of ACOX to benzocyclobutene is29.8 percent based on the recovered organic layer.

EXAMPLE 3

10.1 Grams per minute of liquid o-xylene at ambient temperature, 95g/min of low pressure steam superheated to 180° C., 6.5 g/min ofchlorine gas at ambient temperature, and 50 cm³ /min of nitrogen at 20°C. and 0.98 atm are initially fed concurently through different entryports into the top of the quartz preheater of Example 2. The quartzpreheater is heated to 550° C.

After the feeds are heated and vaporized in the preheater, they are fedto the top of the fluidized bed reactor of Example 2. The reactor ismaintained at substantially atmospheric pressure and at a temperaturebetween 652° C. and 662° C. The calculated average partial pressures ofo-xylene and chlorine are 13 mm and 25 mm mercury, respectively. Thefeeds pass through the coiled reactor. The reactor effluent is condensedand cooled in a shell-and-tube heat exchanger. The condensed effluent iscollected in a product receiver.

After 50 minutes, the feeds to the preheater are stopped. The collectedeffluent forms an organic layer and a wastewater layer in the productreceiver. The organic layer is separated by decantation and analyzed bygas chromatography, mass spectroscopy (GC/MS) and Fourier transforminfrared spectroscopy (GC/FTIR). The analysis is as follows:

    ______________________________________                                                         GC Area                                                      Compound         (Percent)                                                    ______________________________________                                        o-xylene         46.1                                                         ACOX             23.9                                                         benzocyclobutene 10.2                                                         other            19.8                                                         ______________________________________                                    

The analysis shows that o-xylene can react with chlorine in situ to formACOX, and that ACOX can further react to form benzocyclobutene.

Upon repeating the procedures of this example and Examples 1 and 2 withother benzenes and naphthalenes, similar excellent results are obtained.

What is claimed is:
 1. An improved process of preparing a cyclobutareneby pyrolyzing a benzene or a naphthalene substituted with any ofhalomethyl, hydroxymethyl, acetoxymethyl, or trifluoroacetoxymethyl andeither methyl or substituted methyl ortho thereto having at least onehydrogen on the alpha carbon; wherein the improvement comprisesconducting the pyrolysis in the presence of an amount of steam effectiveto substantially reduce the partial pressure of the pyrolyzing compound.2. The process of claim 1 wherein halomethyl is chloromethyl.
 3. Theprocess of claim 1 wherein substituted methyl is lower alkyl orhalomethyl.
 4. The process of claim 3 wherein halomethyl ischloromethyl.
 5. The process of claim 1 wherein the aromatic ring of thepyrolyzing compound is further substituted with at least one substituentstable to the pyrolysis conditions.
 6. The process of claim 5 whereinthe stable group is methyl, methoxy, methoxycarbonyl, nitro, chloro,bromo, or iodo.
 7. The process of claim 1 wherein the cyclobutareneprepared is benzocyclobutene and the pyrolyzing compound isα-chloro-ortho-xylene.
 8. The process of claim 1 wherein theα-chloro-ortho-xylene is prepared in situ by reacting ortho-xylene withchlorine.
 9. The process of claim 1 wherein the mole ratio of steam topyrolyzing compound ranges from about 5:1 to about 100:1.
 10. Theprocess of claim 9 wherein the mole ratio of steam to pyrolyzingcompound ranges from about 19:1 to about 40:1.
 11. The process of claim1 wherein the reaction temperature ranges from about 400° C. to about800° C.
 12. The process of claim 11 wherein the reaction temperatureranges from about 550° C. to about 700° C.
 13. The process of claim 1wherein the total reaction pressure ranges from about 0.1 atmosphere tosubstantially atmospheric.
 14. The process of claim 13 wherein the totalreaction pressure is substantially atmospheric.
 15. The process of claim1 wherein the pyrolysis occurs in a tubular reactor with a length todiameter ratio greater than 10:1.
 16. The process of claim 15 whereinthe pyrolyzing compound is vaporized and combined with the steam beforeentering the reactor.
 17. The method of claim 15 wherein the reactor isheated in a fluidized bed of fine powder.
 18. The method of claim 15wherein the pyrolyzing compound is vaporized and preheated with thesteam to near reaction temperature in a preheater before entering thereactor.
 19. The process of claim 1 wherein the yield of thecyclobutarene prepared is greater than 20 percent.